1. Field of the Invention
This invention relates to a seek control apparatus, a seek control method and a storage apparatus for performing a seek control of a head.
2. Description of the Related Art
Heads are required to be positioned more rigorously accurately than ever in response to the increase in recent years of the number of TPI (tracks per inch) in HDDs (hard disk drives). On the other hand, such heads are required to be accelerated more sharply than ever for seek control from the viewpoint of realizing high speed seek operations.
Note that, the conventional art that relates to the present invention includes methods and apparatus for controlling accesses to recording disks for the purpose of realizing high speed random accesses to recording disk (see, inter alia, Pat. Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 9-185864).
However, as a head is accelerated sharply, a problem of residual oscillations and a consequent problem of write off track can arise. Residual oscillations are oscillations of the position of a head immediately after the end of a seek control process. Such oscillations adversely affect the accuracy of head positioning control. FIG. 10 of the accompanying drawings is a graph showing residual oscillations as an example. In FIG. 10, the horizontal axis indicates time and the vertical axis indicates the head position. A track center and two write off track slices (+ side and − side) are also shown in FIG. 10. The track center refers to a target position and the two write off track slices refer to the + side positional limit and the − side positional limit of the head.
As long as the head position oscillates within the write off track slices, there does not arise any problem of degradation of performance of the head. However, once the head position goes beyond either of the write off track slices, the head faces write prohibition and is subjected to a retry process (of trying to write once again after a full turn of the corresponding disk) to degrade the performance thereof.
Now, the relationship between the block length and performance degradation will be discussed below. FIG. 11 of the accompanying drawings is a timing chart illustrating an example of relationship between a servo gate and a write gate in a 1-block write operation. Similarly, FIG. 12 of the accompanying drawings is a timing chart illustrating an example of relationship between a servo gate and a write gate in a 35-block write operation. In FIGS. 11 and 12, the upper row indicates the waveform of the servo gate and the lower row indicates the waveform of the write gate. The servo gate is a window that is opened in synchronism with the timing of demodulation of the servo pattern arranged on a medium, whereas the write gate is a window that is opened at the time of a write operation conducted on the medium. The servo gate and the write gate are never opened at the same time and the write gate is opened for a plurality of times between two consecutive open periods of the servo gate. The first through seventh servo frames are shown in FIGS. 11 and 12. In the instance of FIG. 12, data of 35 blocks are written over seven servo frames on the medium.
In the instance of the 1-block write operation illustrated in FIG. 11, it is possible to end the write operation without requiring any retry process when the position obtained as a result of servo demodulation of the first servo frame is found within the write off track slices. However, in the instance of the 35-block write operation illustrated in FIG. 12, all the frames including the first through seventh servo frames are required to be found within the write off track slices and a retry process is required if either of the slices is overrun. In other words, the probability of the necessity of executing a retry process rises as the block length to be written increases for the same waveform of residual oscillations.